我可以为 python 中的 if 循环传递我的自定义条件语句吗？

Question

Suppose i have a list in python which contains many words. Now I want to print elements in list on basis of a condition, which I have to take from input. For example, I want all elements which start with 'a' sometimes, or elements which end with 'l'.

So, I want an applicable method to execute this programme:

a=['','apple','ball','cat']
condition = input()
for i in a :
     if condition:
          print i

---

where condition is an expression and I need programme to parse it as a expression instead of string.

Answer 1

Because you handle strings, you can pass a regex as an input.

It gives you:

1. One generic code - Don't handle if s and specific conditions for each case
2. Powerful solution for user - Provides many options to be used, almost no limitations for the user to query.

Do something like:

import re

a=['','apple','ball','cat']
regex = re.compile(input())

# filter only the strings in 'a' which match the given pattern
matches = filter(lambda x: regex.match(x), a)
for i in matches:
    print(i)

Some examples:

input : '^a[az]*$' will match only 'apple'

input : '^[az]*ll' will only match 'ball'

Answer 2

It can be done in this way:

1. Create a function which evaluate the condition, and return True or False if it pass or fail.

2. In for loop use call this function with given input.

Code

def condition_function(argument, condition):
      # check condition for argument here

      # put code to check the condition true ore not
      # result is true or false indication argument satisfy the condition

      if result is True:
               return True
      else:
           return False


a=['','apple','ball','cat']
condition = input()
for i in a :
     if conditions_function(i, condition):
           print i

Answer 3

You can use the str.startswith(letter) method:

a=['','apple','ball','cat']
condition = input()
for i in a :
     if i.startswith("a") or i.endswith("i"):
          print i

Also, what is "condition" input for?

Answer 4

Yes, you can. Use RegEx python library to compile the string and you are good to go.

import re
a = ['', 'apple','nike', 'cat']
expression = input()
r = re.compile(expression)
print(expression)
for ain in a:
    if(re.findall(r,ain)):
        print(ain)